Ion implantation is a physical process that is employed in semiconductor device fabrication to selectively implant dopant into semiconductor and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and semiconductor material. For ion implantation, dopant atoms/molecules are ionized, accelerated, formed into a beam, analyzed, and swept across a wafer, or the wafer is swept through the beam. The dopant ions physically bombard the wafer, enter the surface and come to rest below the surface, at a depth related to their energy.
Referring to FIG. 1, the system 100 includes an ion source 102 for producing an ion beam along a beam path. A beamline assembly 110 is provided downstream of the ion source 102 to receive a beam therefrom. The beamline system 110 may include (not shown) a mass analyzer, an acceleration structure, which may include, for example, one or more gaps, and an angular energy filter. The beamline assembly 110 is situated along the path to receive the beam. The mass analyzer includes a field generating component, such as a magnet, and operates to provide a field across the beam path so as to deflect ions from the ion beam at varying trajectories according to mass (e.g., charge to mass ratio). Ions traveling through the magnetic field experience a force which directs individual ions of a desired mass along the beam path and which deflects ions of undesired mass away from the beam path.
A process chamber 112 is provided in the system 100, which contains a target location that receives the ion beam from the beam line assembly and supports one or more workpieces 114 such as semiconductor wafers along the path for implantation using the final mass analyzed ion beam. The process chamber 112 then receives the ion beam which is directed toward a workpiece 114. It is appreciated that different types of process chambers 112 may be employed in the system 100. For example, a “batch” type process chamber 112 can simultaneously support multiple workpieces 114 on a rotating support structure, wherein the workpieces 114 are rotated through the path of the ion beam until all the workpieces 114 are completely implanted. A “serial” type plasma chamber 114, on the other hand, supports a single workpiece 114 along the beam path for implantation, wherein multiple workpieces 114 are implanted one at a time in serial fashion, with each workpiece 114 being completely implanted before implantation of the next workpiece 114 begins. The process chamber 112 may also include a scanning apparatus for moving the beam with respect to the workpiece, or the workpiece with respect to the beam.
Ion sources in ion implanters typically generate an ion beam by ionizing within a source chamber a source gas, a component of which can be a desired dopant element, and extracting the ionized source gas in the form of an ion beam. The ionization process is effected by an exciter which may take the form of a thermally heated filament, a filament heating a cathode (indirectly heated cathode “IHC”), or a radio frequency (RF) antenna.
Examples of desired dopant elements of which the source gas is comprised can include carbon, oxygen, boron, germanium, silicon, amongst others. Of increasing interest is the use of carbon, which can be utilized in many implant steps, for example, material modification. The most common precursor source gases for carbon implants include carbon dioxide and carbon monoxide.
In constructing the ion source chamber illustrated in FIG. 1, refractory metals such as tungsten and molybdenum are commonly used to form the cathode electrodes and interior wall surfaces of the chamber 102. During generation of the carbon ion utilizing a carbon dioxide or carbon monoxide source gas, free oxygen atoms are generated in the ion chamber, and react with the material from which the electrodes, the chamber liners, chamber body and arc slit are constructed. The chamber 102 will react with the free oxygen ions to form tungsten and molybdenum oxides, which build up on these surfaces and detrimentally affect the efficiency of the ion source and poison the chamber 102.
In order to combat such effects, it has been known to run a co-gas with the carbon dioxide source gas to relieve the destructive tendencies of the free oxygen. Co-gases used for this purpose include, amongst others, phosphine (PH3). A co-gas such as phosphine, however, adds gas flow and pressure to the ion source without adding any usable precursor material, as well as being more expensive and toxic than other alternatives.